With development of communication technology, the number of radio access users has been steadily increased and thus there is a limit in a bandwidth of a wireless network. Demands on high-quality multimedia services have increased and quality of service (QoS) needs to be guaranteed in order to successfully transmit real-time multimedia data such as VoIP or real-time video. In addition, in a situation in which demands on mass multimedia services has increased, in order to provide predetermined QoS at low cost, band efficiency should be maximized. For band efficiency improvement, average throughput of a cell and a maximum radio data transfer rate should be increased. Accordingly, Multiple-input multiple-output (MIMO) technology has been proposed.
Multiple-input multiple-output (MIMO) technology refers to a method of employing multiple transmit antennas and multiple receive antennas so as to improve transmission/reception data efficiency, unlike a conventional method using one transmit antenna and one receive antenna. That is, the MIMO technology refers to a technology of utilizing multiple antennas in a transmitting end and a receiving end so as to increase capacity and improve performance and may also be referred to as a multi-antenna technology.
In the MIMO technique, a single antenna path is not used for receiving one message. Instead, in the MIMO technique, data fragments received via several antennas are collected and combined so as to complete data. If the MIMO technique is used, a data transfer rate may be improved in a specific range or system coverage may be increased with respect to a specific data transfer rate. In addition, this technique is a next-generation mobile communication technique which may be widely used in a mobile communication terminal, a repeater and the like. Therefore, the MIMO technique is attracting considerable attention as a next-generation technique which can overcome a limit in transmission amount of conventional mobile communication using a single antenna by data communication extension.
In general, when a transport channel is in a deep fading state, if another version or a replica of a transmitted signal is not additionally transmitted, it is difficult for a receiver to determine a transmitted signal. A source corresponding to another version or a replica of a transmitted signal is referred to as diversity, which is a very important factor contributing to reliable transmission of a wireless channel.
Use of diversity may maximize data transfer capacity or transfer reliability. A system for performing diversity using multiple transmit antennas and multiple receive antennas is also called a MIMO system or a multi-antenna system.
In order to overcome performance deterioration by channel fading of wireless communication, research into a spatial diversity scheme using a MIMO system has been conducted.
MIMO provides advantages such as high data transfer rate, low error rate and channel capacity increase by using two or more antennas in a transmitter and a receiver.
However, in spite of the advantages of the MIMO system, it is impossible to implement a MIMO system in uplink due to constraints such as size, weight and hardware complexity.
As an alternative scheme, a cooperative diversity scheme has been proposed. The cooperative diversity scheme can obtain advantages such as spatial diversity gain, error rate decrease and channel capacity increase of a MIMO system even when each terminal has one or more antennas as a minimum in a wireless communication network. The cooperative diversity scheme can obtain the advantages of the MIMO system even when a terminal has one or more antennas, by placing a relay or a femtocell and sharing an antenna of the relay and resources such as a frequency band between peripheral terminals so as to form a virtual MIMO system.
The cooperative diversity scheme uses a separate relay.
However, there is a need for a method of performing cooperative communication using terminals without a separate relay and a method of determining terminals for transmitting data using a cooperative diversity scheme with another terminal. Thus, an inter-terminal cooperative communication scheme, that is, a client cooperation (cooperative communication) scheme, has been proposed.
The client cooperation scheme refers to a method of enabling two or more terminals each having a single or multiple antennas to share resources thereof such that data reaches a destination via multiple paths, that is, a communication method of enabling any one terminal to transmit data to another terminal and a base station or only another terminal and enabling another terminal to transmit the data to the base station.